This invention relates generally to control systems for controlling the power factor of AC (alternating current) electric power supply or distribution systems and, specifically, to such control systems wherein capacitors are switched in and out of the power supply system as needed to effect power factor compensation.
AC electric power supply or distribution systems tend to undergo undesirable reactive power changes when subjected to large reactive draws caused by connection and disconnection of dynamic reactive loads. Reactive power flow also increases the losses of the power system, resulting in a higher cost of power delivered to the user. It is necessary, therefore, to provide means for correcting the power factor on the line so that other electrical equipment on the power line is not disturbed and losses are minimized. Capacitor based systems for controlling the power factor of a supply or distribution system are known in the art. Such prior art capacitor based systems utilize voltage level, current level, load demand, and vars as the sensed parameters.
Voltage responsive controls may or may not provide adequate power factor control. Capacitor banks located close to the substations do not have a large effect on the voltage when switched on and off. However, these banks do effect the power factor sensed at the substation. In contrast, banks located a long distance from the substation will have a significant effect on the local voltage when switched on and off, therefore, voltage control systems work well at these remote locations.
Current responsive controls are applied under the assumption that when load is heavy, the voltage will be low and power factor will be poor. Seasonal variations in load mix may make this assumption invalid. Summer AC load may be ideally suited to this type of control, however, in the winter, the same location may have a heavy load that is primarily resistance heating with unity power factor.
Vars sensing controls respond to the var level on the circuit. However, during periods of heavy resistive loading, the capacitors will not be switched on unless the vars control includes a low voltage override unit. This may result in poor voltage profile along the line. During periods of moderate load level, the power factor may be excessively lagging but the var level may not be high enough to switch the bank on. Also during periods of heavy load, the power factor may be acceptable, however, the var level could exceed the var control setting. This will cause the capacitor bank to be on, regardless of the voltage level. If the voltage is high during this period, the var control could cause an overvoltage condition.